Known HID lamp circuits operate in various frequency ranges. Low frequency switching ballast circuits (square wave), such as those which operate in the range of 100-200 Hz, require ignition circuits and complex constant power control. Additionally, the physical size of the components required in a low frequency implementation is restrictively large.
Middle frequency switching ballasts, such as those operating in the frequency range of 10-100 kHz with a resonant ignition, require complex control circuits for arc instability detection and frequency modulation. An example of such a circuit is described in U.S. Pat. No. 5,623,187 issued on Apr. 22, 1997 to Caldeira et al. The system described in this patent includes a complex circuit required to detect and adjust the operating frequency of the inverter circuit to avoid acoustic resonance of the discharge arc.
Other references, such as U.S. Pat. No. 5,719,474 issued on Feb. 17, 1998 to Vitello and U.S. Pat. No. 5,850,127 issued on Dec. 15, 1998 to Zhu et al. disclose lamp circuits which operate at up to 100 kHz. The circuit of Vitello requires a microprocessor to control the inverter circuit, and neither patent utilizes the leakage inductance inherent in a transformer output as a current regulating device.
U.S. Pat. No. 5,877,592 issued Mar. 2, 1999 to Hesterman et al. describes a ballast circuit which operates in the 40-50 kHz range and includes inductive elements in the lamp connections. The system relies upon a timer circuit to switch from the preheat phase, however, and does not operate in a manner in which the inverter can operate uncontrolled by its control circuit until the lamp heats up.
All of the foregoing patents describe electronic ballast circuits for lamps which operate in a frequency range of up to 100 kHz. None of the circuits operate in the range of 300 kHz, and none are designed in conjunction with a transformer output circuit with high leakage inductance and an operating frequency of the inverter which exceeds the resonant frequency of the tank circuit.